Think JSFiddle. LINQPad is pretty sweet for fast C# with similar options. Is there anything like LINQPad that runs in a browser?
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You could try www.compilr.com. According to their website, Compilr is an online IDE, allowing you to write and compile code for a variety of platforms from any major web browser.
I've tried it out, it works fairly well. It supports C#, Ruby, C, Java and others.
I don't know of anything like that, but the reason JSFiddle is practical is because the code is run natively on the browser, not the server. In order to do something similar for C# code, you would most likely need to do so on the server or use a browser plugin.
A plugin solution would defeat the purpose of even being browser-based because it would require an installation and limit the platforms it can run on. A server-based solution would have many inherent risks unless it were heavily sandboxed.